1. Field of the Invention
The present invention relates to an improved laser surgical method and more particularly relates to an improved method of reshaping or sculpting the outer surface of a patient's cornea wherein a masking system includes a single or multi-layer coated substrate used to control the power transmission of a laser that is used in conjunction with the masking system. More particularly, the present invention relates to a laser surgery masking system for sculpting a patient's cornea using a mesh constructed to control power distribution through its substrate, preferably in the form of a grid having defined variable sized patterns etched through a reflective substrate over a transparent or translucent material (e.g. fused silica) wherein the pattern can be etched onto a deposited metal coating or other reflective or totally absorbent coating.
2. General Background
The corneal tear/air interface is the major refracting surface in the eye with an average convergent refractive power of 48.8 D. The human cornea has an average thickness of about 0.52 mm, and a transmittance cutoff wavelength of 300 nm. In recent years considerable advancement in low wavelength lasers has opened frontiers in refractive surgery.
Corneal topography can be used to estimate deformations and abnormalities of the corneal surface prior to refractive surgery. Low wavelength lasers could make refractive surgery a cost effective and common surgical procedure.
Various surgical techniques have been proposed for reprofiling of the corneal surface. One common technique is radial keratotomy in which a set of four to eight radial incisions are made in the eye to flatten the curvature of the cornea.
Another suggested technique employed the use of a low wavelength excimer laser to perform controlled ablative photodecomposition. Although altering the curvature of the outer corneal surface has been effective in correcting spherical myopia, the limitation with current laser technology is the inadequacy of corrective surgery in aspheric, irregularly astigmatic corneal surfaces.
Some patents have been issued which relate generally to the sculpting of a patient's cornea. For example, U.S. Pat. No. 4,911,711 entitled "Sculpting Apparatus for Correcting Curvature of the Cornea" provides ultraviolet-laser sculpting of the cornea to achieve optical correction through a newly shaped anterior surface. The apparatus subjects the laser beam to certain shaping and homogenizing operations prior to any attempt to specially characterize the beam for a particular sculpting procedure. In a preferred embodiment, the shaping and homogenizing operations present a tolerably homogeneous beam of enlarged dimension, so that specialty-characterizing may proceed on a dimensional scale that is greater than the corresponding dimension of ultimate surgical delivery to the eye, thereby enabling greater control of the quality of specialty characterizing. Provision is made for selectively monitoring the quality of the homogeneity and/or the specially characterized beam, with further provision for automatic cutoff of laser beam delivery to an eye in the event that quality is not within the predetermined limits of tolerance. And preferably, all beam shaping, homogenizing and characterizing operations proceed in a controlled environment which precludes ozone development and thus minimizes the beam degrading effect of ozone and particulates or other contaminants.
U.S. Pat. No. 4,994,058 entitled "Surface Shaping Using Lasers" issued to Anthony Raven et al. provides a laser system and masking apparatus for reprofiling surfaces, such as corneal surfaces. The system includes a laser and a mask disposed between the laser and the surface to be reprofiled, the mask providing a predefined profile of resistance to laser radiation, such that upon irradiation, part of the radiation is selectively absorbed and part is transmitted to the surface in accordance with the masked profile, to selectively erode the surface. The masking apparatus disclosed is a mask to be affixed to the surface, or may include a support structure to support and position the mask above the surface. The resistance profile is stated as created by varying the thickness or the composition of the mask.